Project Summary/Abstract Our long-term goal is to understand how the control of protein synthesis in skeletal muscle can be manipulated for preventive and therapeutic purposes. The objective here is to gain an understanding of how to prevent and/or reverse inactivity-induced loss of skeletal muscle mass. Inactivity encompasses a number of conditions that lead to loss of muscle mass including sedentary lifestyles, chronic bed rest, casting, limb suspension, and immobilization. Each of these inactivity-related conditions is thought to share a common feature, i.e. a delayed and reduced magnitude in the response of skeletal muscle protein synthesis to an anabolic stimulus, which has been referred to as anabolic resistance. Our central hypothesis is that anabolic resistance is due to dysregulation of the molecular mechanisms through which nutrients, specifically the amino acid leucine, act to mediate regulation of the mechanistic target of rapamycin complex 1 (mTORC1) resulting in defects in the efficiency and capacity of protein synthesis as well as the function of the proteasome to maintain proteostasis in skeletal muscle. Our hypothesis has been formulated on the basis of our past accomplishments in elucidating the regulation of mTORC1 signaling and protein synthesis in skeletal muscle under a variety of physiological and pathophysiological conditions. To test our hypothesis, we will pursue an experimental protocol involving model systems ranging from intact rats to intact tissues to cell culture, as well as cutting-edge technologies for analyzing mRNA translation and protein phosphorylation sites. The proposed studies encompass the following three specific aims: (1) identify molecular components required for the selective action of leucine in mediating the activation of mTORC1; (2) delineate the regulation of Sestrin2 phosphorylation and its role in mediating anabolic resistance in disuse atrophy of skeletal muscle; and (3) establish the relationship between the activation state of mTORC1 and the response of its targets that control the efficiency and capacity of protein synthesis, as well as the function of the proteasome, in maintaining proteostasis in skeletal muscle. Overall, we expect the proposed research to reveal molecular targets that can likely be manipulated pharmacologically resulting in new and innovative approaches to the prevention and treatment of muscle wasting conditions.